1. Field of the Invention
The invention relates to transmission lines and to transitions between different kinds of transmission lines. More particularly, the invention relates to a transition between a "stripline" and a "microstrip" transmission line.
2. Prior Art
Increasing use of high frequency circuitry in electronic equipment has led to simpler and more readily manufactured forms of r.f. propagating elements. Laboriously manufactured waveguides and coaxial lines have given way wherever possible, to lower cost and more easily manufactured stripline and microstrip transmission lines.
Stripline and microstrip transmission lines may be formed using printed circuit board (PCB) materials and processing. The starting material for typical stripline and microstrip transmission lines is a low loss, usually low dielectric constant material with good mechanical properties in sheet form coated on one or both sides with a continuous conductive layer. The conductive layer is selectively removed to achieve desired r.f. propagation paths by a highly automated printing process.
In wave propagating circuitry, two distinctive needs have arisen for "active" circuitry on the one hand and "passive" r.f. combining or distribution circuitry on the other hand. The solution to these needs has led to the large scale use of the two printed transmission lines mentioned above.
The "active" circuitry, which may include passive circuit components such as inductors, capacitors, resistors, discrete semiconductors, and monolithically integrated microwave integrated circuits, often requiring interconnections in a hybrid format, is usually best connected by "microstrip". A microstrip employs a single finite width conductor disposed on a layer of dielectric material over an "infinite" width conductor acting as a ground plane to propagate the r.f. signal. The microstrip configuration allows circuit components of variable thicknesses and requiring interconnection to be disposed on the top surface of the dielectric layer without the interference of an overlaying ground plane.
On the other hand "passive" r.f. combining or Distribution circuitry, for instance that used in beam forming for an antenna array, has a different requirement. This circuitry requires shielded transmission paths and complex branching. In "stripline", a single finite width conductor is disposed between two dielectric layers each having an outer ground plane. Appropriate dimensioning within the stripline assembly provides adequate internal isolation between distinct signal paths, while the outer ground planes provide external shielding comparable to that of a coaxial line or waveguide. The stripline is flexible in its applications and may be used to form delay lines, branching networks, circulators and other complex microwave interconnections.
The prevalence of both types of printed transmission lines in modern electronic equipment, and the usually complementary applications of the two type of lines has developed the need for both types of lines in the same electronic equipment and has created a need for a simple, easily manufactured transition from one form of transmission line to the other.